1. Field of the Invention
The present invention relates to spatially registering measured values deduced from multiple modes of measuring the same body, such as a patient, for integrated views of the body; and, in particular to elastic registration of combined scanning technologies, such as a hybrid Computer-aided Tomography (CT) Positron Emission Tomography (PET) apparatus, and adaptive elastic registration based on sub-volume division.
2. Description of the Related Art
Different sensing systems are widely known and used for non-invasively probing the interior structure of bodies. For example, X-rays and X-ray-based computer-assisted tomography (CT), nuclear magnetic resonance (NMR) and NMR-based magnetic resonance imagery (MRI), acoustic waves and acoustics-based ultrasound imagery (USI), positron emissions and positron emission tomography (PET), and optical waves have all been used to probe the human body and bodies of other animals. Some have been used to probe non-living entities such as machinery, buildings, and geological features. Full and partial body scans can be constructed by assembling sequences of images and other output produced by these systems. Each body scan produced by a sensing system is herein called a measurement mode of the target body. In general, a measurement mode produces a two-dimensional (2D) image, a three dimensional (3D) volume based on a set of images with the third dimension being either a spatial dimension or time, or a full four dimensional (4D) volume based on three spatial dimensions and time.
Various sensing systems respond to different physical phenomena, and hence provide different information about structures within the target body. The range of measurable quantities increases as tracers of various kinds are injected into the body to highlight particular structures or functions within the body when the non-invasive measurements are made. Great insight into the structural and functional properties inside the target body can be achieved by fusing the information from multiple measurement modes of the same body, either with the different sensing systems or with the same sensing system at different times during the target body's stages of operation.
For many applications that attempt to fuse the information available in multiple measurement modes, the images or volumes produced by the various measurement modes need to be matched, i.e., spatially or temporally registered, or both, so that corresponding parts of the measurement products can be compared. In some cases, the registration is easy because the sensing systems generate products of the same time and space resolution and scale while the body was stationary between measurements. In general however, the measurement product of one system has a different scale or resolution in time or space or both, with a different response to each volume element, and the target body is not stationary over the time scale of the various temporal resolutions. Consequently, more sophisticated registration of measurement products is needed.
For example, the information in CT scans provides tissue arrangements and the information in PET scans provides cellular function. However, CT body scans are produced by snapshots of a patient in one pose during a maximum inspiration phase of a patient's breathing cycle; while PET body scans are produced by measurements at lower spatial and temporal resolution, encompassing several minutes that represent an average over the patient's breathing cycle. In some circumstances the patient is in a different pose for the PET scan than for the CT scan.
An approach to automatically register PET and CT images, which addresses these issues was published by Vivek Walimbe, Vladimir Zagrodsky, Shanker Raja, Bohdan Bybel, Mangesh Kanvinde and Raj Shekhar, “Elastic registration of three-dimensional whole body CT and PET images by quarternion-based interpolation of multiple piecewise linear rigid-body registrations,” Medical Imaging 2004: Image Processing, edited by J. Michael Fitzpatrick, Milan Sonka, Proceedings of SPIE Vol 5370, pp. 119-128, SPIE, Bellingham, Wash., February 2004 (hereinafter Walimbe) and is hereby incorporated by reference as if fully set forth herein. Walimbe makes use of normalized mutual information (NMI) based rigid-body registration adapted in a procedure described in R. Shekhar and V. Zagrodsky, “Mutual information-based rigid and nonrigid registration of ultrasound volumes,” IEEE Trans. Med. Imaging, vol. 21, pp 9-22, 2002 (hereinafter Shekhar I), the entire contents of which are hereby incorporated by reference as if fully set forth herein. The Walimbe approach first does a rigid-body registration for a floating 3D PET scan relative to a reference 3D CT scan, then breaks one scan into multiple 3D sub-volumes and does a rigid body correction on each sub-volume. The process is continued at further levels of sub-dividing 3D volumes until a minimum number (4096) of volume elements (voxels) is no longer available in a sub-volume. A voxel is a spatial element that has a single measurement value. The net sub-volume rigid registrations are interpolated onto a fine mesh grid appropriate to the CT spatial resolution using bi-linear interpolation to provide an elastic registration for the whole PET scan. A significant innovation of the Walimbe approach is the inclusion of 3D angular registrations at each sub-volume, and interpolation of angles using quaternion angular coordinates. The approach is applicable and can be extended readily to both 2D and 4D scans as well.
While the approach of Walimbe both speeds and improves registration accuracy compared to prior approaches, and is suitable for many purposes, the approach does have some remaining deficiencies. For example, using fixed values for some parameters of the method, some sub-volumes can not be used reliably in some circumstances because of low information content compared to a global histogram of mutual information.
In another approach, PET sensors and CT sensors are combined in a hybrid apparatus with fixed geometries relative to a patient couch and uses mechanical registration to reduce the variability in time, patient condition, and patient position between the different measurement modes. While suitable for reducing variability, important variability is not eliminated, and patient breathing differences and shifting continue to introduce misalignments of features of interest.
Based on the foregoing, there is a clear need for techniques to perform interpolation of sub-volume registrations that do not suffer the deficiencies of prior art approaches and are adaptive to the particular measurement modes being registered.
In particular, there is a need for techniques able to further reduce the residual misalignments between multiple sensors in a combined apparatus.